This application claims priority to an application entitled xe2x80x9cMethod and Apparatus for Link Adaptation in a Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Jun. 27, 2000 and assigned Serial No. 2000-35792, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a link adaptation apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
In a mobile communication system using radio channels, an attenuation of a radio signal varies depending on a distance between an access network (AN) and an access terminal (AT), and shadowing. Further, the radio channels used in the mobile communication system experience considerable interference between signals, and fading. Therefore, a carrier-to-interference ratio (hereinafter, referred to as xe2x80x9cC/Ixe2x80x9d for short) is subjected to severe variations depending on the condition of the radio channels. A link adaptation technique has been proposed to increase throughput of a channel by adjusting a data rate according to the channel condition (or C/I). The data rate is determined depending on a coding rate and a modulation mode. When the C/I is high, a higher coding rate and a high-level modulation mode are used to increase the data rate. On the contrary, however, when the C/I is low, a lower coding rate and a low-level modulation mode are used to increase reliability of the channel.
In order to transmit data while maintaining the channel reliability according to the C/I, a receiver and a transmitter in the mobile communication system operate as follows. The receiver predicts a variation of the next channel based on the C/I to determine a data rate, and then, transmits the determined data rate information to the transmitter. The transmitter then assigns a data rate according to the data rate requested by the receiver and maintains a transmission power level constantly to a reference level.
Measurement of the C/I and assignment of the data rate will be described below with reference to an HDR (High Data Rate) forward link proposed in 3GPP2 (3rd Generation Partnership Project 2), by way of example. In the foregoing description, the transmitter corresponds to an access network (AN), while the receiver corresponds to an access terminal (AT). The HDR physical layer based on the link adaptation technique supports 13 transmission modes, which are determined by a combination of three modulation modes of QPSK (Quadrature Phase Shift Keying), 8 PSK (8-ary Phase Shift Keying) and 16 QAM (16-ary Phase Shift Keying), three coding rates of xc2xc, xe2x85x9c and xc2xd, and the number of slots where a packet is repeated. The transmission power level of HDR system is the maximum transmission power level.
FIG. 1 illustrates a transmission/reception timing diagram of forward and reverse links in an HDR system. Referring to FIG. 1, the forward and reverse packets each include 2048 chips per slot. Further, each slot includes one pilot channel per half slot (xc2xd slot), and each pilot channel includes 96 chips. Such a pilot channel is usually transmitted at the same power as that of a traffic channel. Therefore, the HDR system estimates a C/I of the traffic channel by measuring a C/I of the pilot channel. That is, the receiver measures a C/I value of the pilot channel and determines a data rate depending on the measured C/I value. The receiver transmits the determined data rate information to the transmitter. In the HDR specification, the data rate information transmitted to the transmitter is referred to as xe2x80x9cDRC (Data Rate Control).xe2x80x9d The DRC is transmitted over a DRC channel, and represented by a 4-bit DRC symbol.
FIG. 2 illustrates puncturing patterns of a pilot channel, a DRC channel and an RRI (Reverse Rate Indicator) channel for the reverse link of the HDR system. Here, the RRI channel is used to transmit data rate information of a reverse traffic channel. With reference to FIG. 2, a description will be made herein below of a structure of each channel for the reverse link.
First, a process for constructing a DRC symbol transmitted over the DRC channel will be described. The DRC symbol transmitted over the DRC channel is block-encoded with each code of (8,4,4) bi-orthogonal code on a one-to-one basis according to a data rate. Thereafter, the access terminal repeats an 8-bit DRC symbol transmitted over the reverse link, once every bit. Further, the access terminal spreads the repeated DRC symbol with an 8-bit Walsh code indicating a sector to which the access terminal belongs. The spread DRC symbol is spread again with a 4-bit Walsh code, constructing a DRC symbol comprised of a total of 512 chips. The 512-chip DRC symbol is repeated once again, so that each slot includes 1024 chips assigned to the DRC channel. The DRC chips are divided into 16 64-chip TDM (Time Division Multiplex) slots, and transmitted together with the pilot and RRI channels on a TDM basis, as shown in FIG. 2. That is, the DRC chips are alternately inserted starting from the first TDM slot. An RRI symbol is inserted once in the second TDM slot of a 2048-chip slot. Further, pilot symbols are inserted in the TDM slots into which the DRC symbols are not inserted, thereby constructing one slot.
Now, a description will be made regarding a process of determining a data rate depending on a C/I of the forward channel and a process of transmitting the DRC over the reverse channel in the HDR system employing the link adaptation technique.
The HDR system supports several predefined data rates, and each data rate has a unique coding rate and a unique modulation mode. Further, the receiver includes a C/I table for storing a C/I threshold satisfying a specific packet error probability at every data rate. Therefore, the receiver measures a C/I value of the pilot channel among the forward channels, and compares the measured C/I value with the C/I thresholds stored in the C/I table. The receiver searches the largest one of the C/I thresholds that is smaller than the measured C/I value, and determines a corresponding data rate as an acceptable data rate. The receiver transmits the determined data rate information to the transmitter over the DRC channel out of the reverse channels.
There exists an interval between the adjacent C/I thresholds stored in the C/I table. Therefore, even though a C/I threshold closest to the C/I value is selected, there exists a difference between them. This difference becomes surplus transmission power on the C/I and the packet error probability. Therefore, transmitting a transmission channel at a data rate associated with the selected C/I threshold causes an unnecessary waste of transmission power.
Table 1 below illustrates a C/I table in which C/I thresholds are stored, by way of example. When the measured C/I value is xe2x88x9213 dB, the receiver selects a C/I threshold of xe2x88x9215 dB in accordance with Table 1. In this case, there exists a difference of 2 dB between the measured C/I value and the selected C/I threshold.
The C/I thresholds given in Table 1 satisfy a receiving error rate at each data rate. Therefore, the receiving error rate is duly satisfied even for the C/I value of xe2x88x9215 dB. However, when the C/I value is xe2x88x9213 dB, there occurs surplus power of xe2x88x922 dB, causing an unnecessary waste of power during data transmission. In addition, the surplus power causes interference between channels and a waste of bandwidth.
In conclusion, since the HDR system employing the link adaptation technique determines a data rate by comparing the C/I value with predetermined C/I thresholds, there occurs a difference between the actual C/I value measured and the C/I threshold that becomes a criterion for determining the data rate. The difference causes an unnecessary waste of power at the transmitter, increases interference between channels in the radio environment, and causes a waste of bandwidth.
It is, therefore, an object of the present invention to provide an apparatus and method for reducing surplus transmission power in a mobile communication system employing link adaptation.
It is another object of the present invention to provide an apparatus and method for indicating a data rate and a transmission power level in a mobile communication system employing link adaptation.
It is further another object of the present invention to provide an apparatus and method for transmitting a C/I value of a forward channel to a transmitter in a mobile communication system employing link adaptation.
According to one aspect of the present invention, there is provided a method for determining a forward data rate and a forward transmission power level in a mobile communication system. An access terminal measures a C/I of a forward pilot channel, determines a forward data rate by matching the measured C/I with a reference C/I, creates a difference between the measured C/I and the reference C/I as margin information, and transmits the determined forward data rate and margin information over a reverse transmission channel. Upon receipt of the forward data rate and margin information, an access network decreases a transmission power level by power corresponding to the margin information and performs forward transmission at the forward data rate at the decreased transmission power level.
According to another aspect of the present invention, there is provided a method for determining a forward data rate and a forward transmission power level in a mobile communication system. An access terminal measures a C/I of a forward pilot channel, and transmits the measured C/I over a reverse data rate channel. Upon receipt of the C/I received over a reverse link, an access network determines a forward data rate by matching the measured C/I with a reference C/I associated with a data rate of packet data, determines margin information for determining a forward transmission power level by calculating a difference between the received C/I and the reference C/I when the received C/I is not identical to the reference C/I, creates transmission data associated with the determined data rate, decreases the transmission power level associated with the data rate using the calculated margin information, and transmits the transmission data at the decreased transmission power level.
According to further another aspect of the present invention, there is provided a transmission apparatus for a mobile communication system, for determining a data rate using a C/I and transmitting a forward data rate and a transmission power level over a reverse transmission channel using margin information determined based on a difference between a reference C/I and the C/I. The apparatus comprises: a first multiplexer for time-division-multiplexing the data rate and the margin information; an encoder for encoding an output of the first multiplexer; a spreader for spreading the encoded data rate and margin information; and a second multiplexer for time-division-multiplexing a reverse pilot channel and an RRI (Reverse Rate Indicator) channel to an output of the spreader.
According to yet another aspect of the present invention, there is provided a transmission apparatus for transmitting a C/I in a mobile communication system. The apparatus comprises: an encoder for encoding a measured C/I; a spreader for spreading an output of the encoder; and a multiplexer for time-division-multiplexing an output of the spreader, a reverse pilot channel and an RRI channel.